The use of cell fusion to analyse factors involved in tumour cell metastasis

The cancer cell--leukocyte fusion theory of metastasis

The cause of metastasis remains elusive despite vast information on cancer cells. We posit that cancer cell fusion with macrophages or other migratory bone marrow-derived cells (BMDCs) provides an explanation. BMDCs fused with tumor cells were present in animal tumor xenografts where they were associated with metastases. In myeloma patients, transcriptionally active myeloma nuclei were incorporated into osteoclasts through fusion. In patients with renal cell carcinoma arising poststem cell transplant, donor genes were incorporated in recipient cancer cell nuclei, most likely through fusion, and showed tumor distribution patterns characteristic of cancer stem cells. Melanoma-macrophage hybrids generated in vitro contained chromosomes from both parental partners, showed increased ploidy, and transcribed and translated genes from both parents. They exhibited chemotactic migration in vitro toward fibronectin and exhibited high frequencies of metastasis when implanted in mice. They produced macromolecules that are characteristic of macrophages and known indicators of metastasis (c-Met, SPARC, MCR1, GnT-V, and the integrin subunits alpha(3), alpha(5), alpha(6), alpha(v), beta(1), beta(3)). They also produced high levels of beta1,6-branched oligosaccharides-predictors of poor survival in patients with melanoma or carcinomas of the breast, lung, and colon. We thus hypothesize that such gene expression patterns in cancer are generated through fusion. Tumor hybrids also showed active autophagy, a characteristic of both metastatic cancers and macrophages. BMDC-tumor cell fusion explains epidermal-mesenchymal transition in cancer since BMDCs express mesodermal traits and epithelial-mesenchymal transition regulators (Twist, SPARC, and others). If BMDC-tumor cell fusion underlies invasion and metastasis in human cancer, new approaches for therapeutic intervention would be mandated.

Molecular mechanisms of metastasis

Metastasis formation is an essential aspect of cancer, for which the molecular underpinning has long been subject to debate. Although the organ preference for dissemination is governed by tumor-host interactions on the epigenetic level there is a genetic basis to the ability of cancer cells to disseminate. Metastasis genes encode homing receptors, their ligands, and extracellular matrix-degrading proteinases, which jointly cause invasion and anchorage-independence. They are developmentally non-essential stress response genes that physiologically mediate the homing of immune system cells. Metastatic potential is conferred to cancer cells by aberrant expression or splicing of these genes. Oncogenes act upstream of metastasis genes. In cancer cells, oncogenic signaling activates distinct genetic programs leading to cell cycle progression and invasiveness, respectively. The expression of metastasis genes is regulated by multi-subunit transcription factor complexes. The identification of genes that direct cancer metastasis implicates them as candidate drug targets.

Fusion of tumour cells with bone marrow-derived cells: a unifying explanation for metastasis

The causes of metastasis remain elusive despite vast information on cancer cells. We posit that cancer cell fusion with macrophages or other migratory bone marrow-derived cells (BMDCs) provides an explanation. BMDC-tumour hybrids have been detected in numerous animal models and recently in human cancer. Molecular studies indicate that gene expression in such hybrids reflects a metastatic phenotype. Should BMDC-tumour fusion be found to underlie invasion and metastasis in human cancer, new approaches for therapy would surely follow.

Frequent loss of heterozygosity at 8p22 chromosomal region in diffuse type of gastric cancer

AIM: To study the loss of heterozygosity (LOH) at 8p21-23 locus in diffuse gastric cancer.

METHODS: To evaluate the involvement of this region in gastric cancer, we used eight microsatellite markers covering two Mb of mentioned region, to perform a high-resolution analysis of allele loss in 42 cases of late diffuse gastric adenocarcinoma.

RESULTS: Six of these STS makers: D8S1149, D8S1645, D8S1643, D8S1508, D8S1591, and D8S1145 showed 36%, 28%, 37%, 41%, 44% and 53% LOH, respectively.

CONCLUSION: A critical region of loss, close to the NAT2 locus and relatively far from FEZ1 gene currently postulated as tumor suppressor gene in this region.

Tumor dormancy and the role of metastasis suppressor genes in regulating ectopic growth

Metastasis, or tumor growth in an ectopic site, may occur several years after apparently successful treatment of the primary malignancy. Clinical dormancy is seen in a large number of cancer patients, but once growth in an ectopic site initiates, current adjuvant therapies are inadequate and the majority of patients with metastatic disease will die. Many genes may regulate ectopic growth in a secondary site, including a small subset, termed the metastasis suppressor genes. Investigation into this class of genes holds promise in terms of gaining a greater understanding of tumor dormancy and how the process of metastasis may be naturally inhibited. This review will focus on the role of metastasis suppressor genes in tumor dormancy. Insights into the metastatic process from studies of metastasis suppressor genes may lead to novel targets for antimetastatic therapy through drug-induced reactivation of one or more of these genes and/or their respective signaling pathways.

The microcell mediated transfer of human chromosome 8 into highly metastatic rat liver cancer cell line C5F

AIM: Our previous research on the surgical samples of primary liver cancer with CGH showed that the loss of human chromosome 8p had correlation with the metastatic phenotype of liver cancer. In order to seek the functional evidence that there could be a metastatsis suppressor gene (s) for liver cancer on human chromosome 8, we tried to transfer normal human chromosome 8 into rat liver cancer cell line C5F, which had high metastatic potential to lung.

METHODS: Human chromosome 8 randomly marked with neo gene was introduced into C5F cell line by MMCT and positive microcell hybrids were screened by double selections of G418 and HAT. Single cell isolation cloning was applied to clone microcell hybrids. Finally, STS-PCR and WCP-FISH were used to confirm the introduction.

RESULTS: Microcell hybrids resistant to HAT and G418 were obtained and 15 clones were obtained by single-cell isolation cloning. STS-PCR and WCP-FISH proved that human chromosome 8 had been successfully introduced into rat liver cancer cell line C5F. STS-PCR detected a random loss in the chromosome introduced and WCP-FISH found a consistent recombination of the introduced human chromosome with the rat chromosome.

CONCLUSION: The successful introduction of human chromosome 8 into highly metastatic rat liver cancer cell line builds the basis for seeking functional evidence of a metastasis suppressor gene for liver cancer harboring on human chromosome 8 and its subsequent cloning.

Reduced expression of the metastasis suppressor gene KAI1 in advanced colon cancer and its metastases

BACKGROUND

The expression of the KAI1 gene and its gene product were studied in metastatic and non-metastatic human colorectal cancer to evaluate its role in the metastatic process.

METHODS

KAI1 mRNA and protein expression was examined in 36 primary colorectal carcinomas and 6 liver metastasis using Northern blot and Western blot analyses. Forty-six normal colonic tissue samples served as controls. The exact site of KAI1 expression was analyzed by in situ hybridization and by immunohistochemistry in primary tumors, in the corresponding normal tissues, in lymph node metastases and liver metastases.

RESULTS

Densitometric analysis of Northern blots revealed overexpression of KAI1 mRNA in 87% of colonic cancer tissues in comparison with the corresponding normal colonic tissues. This increase was 9.1-fold in median (P < .001). KAI1 mRNA expression was strongly dependent on tumor stage. Colorectal cancer at stages II and III revealed significantly higher KAI1 mRNA levels than stage IV tumors (P < .03 and P < .015, respectively) or normal controls. In addition, liver metastases showed reduced KAI1 mRNA expression when compared with their corresponding primary tumor. In situ hybridization confirmed the stage-dependent expression results obtained by Northern blots, in which the KAI1 mRNA signal was exhibited almost exclusively in the epithelial cells. Lymph node and liver metastases were largely devoid of KAI1 mRNA. Western blot analysis showed a highly significant increase of KAI1 protein level in stage II cancers in comparison with the normal colon (P < .001) but also in comparison with the more advanced tumor stages III and IV (P < .03) and P < .02, respectively), when metastases were already present. In accordance, KAI1 immunostaining decreased successively with the advance of the tumor stage and was absent in lymph node and liver metastases.

CONCLUSIONS

These data demonstrate that the KAI1 mRNA expression and the KAI1 protein level increase in an earlier tumor stage of colorectal cancer, decrease in advanced stages, and are lost in metastases. The loss of KAI1 might favor the ability of colorectal cancer cells to metastasize.

Identification of inbred mouse strains harboring genetic modifiers of mammary tumor age of onset and metastatic progression

Metastasis is one of the most important and complex processes in human neoplastic disease. A large number of both positive and negative events must occur to permit a tumor cell to colonize a distant site successfully. To identify mouse strains that harbor dominant genetic modifiers of this process, a strain survey was initiated utilizing a transgenic mouse mammary tumor model that exhibits a high incidence of pulmonary metastases. The transgenic animal was bred to 27 different inbred strains of mice and scored for the metastatic organ tropism and metastatic density. Thirteen strains were identified that had a statistically significant reduction in the numbers of pulmonary metastases. In addition, 10 strains were identified that altered the kinetics of induction of the primary mammary tumor. These strains will likely provide useful model systems for the analysis of genetic interactions in the initiation and progression of mammary adenocarcinomas.

Melanoma x macrophage hybrids with enhanced metastatic potential

Studies were conducted on the hypothesis that melanoma metastasis might be initiated through the generation of hybrids comprised of cells of the primary tumor and tumor-infiltrating leukocytes. Fusion hybrids were generated in vitro between weakly metastatic Cloudman S91 mouse melanoma cells and normal mouse or human macrophages. Hybrids were implanted s.c. in the tail and mice were monitored for metastases. Controls included parental S91 cells, autologous S91 x S91 hybrids, and B16F10 melanoma cells. Of 35 hybrids tested, most were more aggressive than the parental melanoma cells, producing metastases sooner and in more mice. A striking characteristic was heterogeneity amongst hybrids, with some lines producing no metastases and others producing metastases in up to 80% of mice. With few exceptions, hybrids with the highest metastatic potential also had the highest basal melanin content whereas those with the lowest metastatic potential were basally amelanotic, as were the parental melanoma cells. A spontaneous in vivo supermelanotic hybrid between an S91 tumor cell and DBA/2J host cell was one of the most metastatic lines. Hybrids with the highest metastatic potential also exhibited markedly higher chemotaxis to fibroblast-conditioned media. Histologically, the metastatic hybrids demonstrated vascular invasion and spread to distant organs similar to that of metastatic melanomas in mice and humans. Thus previous findings of enhanced metastasis in leukocyte x lymphoma hybrids can now be extended to include leukocyte x melanoma hybrids. Whether such hybridization is a natural cause of metastasis in vivo remains to be determined; however the fusion hybrids with genetically-matched parents described herein so closely resembled naturally-occurring metastatic melanoma cells that they could serve as useful new models for studies of this complex and deadly phenomenon.

Loss of expression of a 55 kDa nuclear protein (nmt55) in estrogen receptor-negative human breast cancer

We have identified and characterized a 55 kDa nuclear protein (referred to as nmt55) from human breast tumors and MCF-7, human adenocarcinoma breast cell line, using site-directed monoclonal antibodies. Measurements of estrogen receptors (ER) and progesterone receptors (PR), by ligand binding assays, in cytosols of 63 human breast tumors permitted classifications of these tumors into four phenotypes (ER+/PR+, ER+/ PR-, ER-/PR-, ER-/PR+). Nuclear protein (nmt55) expression in these tumors, as determined from Western blot analyses, showed a statistically significant association (p = 0.001) with tumor hormonal phenotype. Review of the pathologic characteristics of tumors analyzed suggested that lack of nmt55 expression was significantly associated with mean tumor size (p < 0.03), mean ER (p = 0.001) and mean PR (p < 0.002), but was not associated with tumor stage, grade, or type. To further study this protein, we cloned and sequenced a 2.5 kb cDNA using a monoclonal antibody to nmt55. The complete predicted open reading frame encodes a protein with 471 amino acids and a calculated molecular mass of 54,169 Da. The deduced amino acid sequence exhibited unique regions rich in glutamine, histidine, arginine, and glutamic acid. Northern blot analysis of RNA from MCF-7 cells and ER+/PR+ human breast tumors showed a 2.6 kb mRNA. Southern blot analysis suggested the presence of a single copy of this gene. Chromosomal mapping, using fluorescent in situ hybridization (FISH), located nmt55 gene to the X chromosome, region q13. The extensive homology between nmt55 and RNA binding proteins suggested that nmt55 may be involved in hnRNA splicing. The strong association observed between expression of nmt55, tumor hormonal phenotype, mean tumor size, mean ER, and mean PR content suggests that loss of nmt55 expression may be related to events involved in hormone insensitivity, tumor differentiation, and unregulated tumor cell growth and metastases.

KAI1, a metastasis suppressor gene for prostate cancer on human chromosome 11p11.2

A gene from human chromosome 11p11.2 was isolated and was shown to suppress metastasis when introduced into rat AT6.1 prostate cancer cells. Expression of this gene, designated KAI1, was reduced in human cell lines derived from metastatic prostate tumors. KAI1 specifies a protein of 267 amino acids, with four hydrophobic and presumably transmembrane domains and one large extracellular hydrophilic domain with three potential N-glycosylation sites. KAI1 is evolutionarily conserved, is expressed in many human tissues, and encodes a member of a structurally distinct family of leukocyte surface glycoproteins. Decreased expression of this gene may be involved in the malignant progression of prostate and other cancers.

Genetic factors and suppression of metastatic ability of v-Ha-ras-transfected rat mammary cancer cells

Following v-Ha-ras transfection of nonmetastatic dimethylbenz(( a ))anthracene-induced rat mammary cancer (RMC1) cells, occasional transfectants were isolated that acquired high metastatic ability. High metastatic ability is not a simple process regulated by v-Ha-ras p21 levels alone in these v-Ha-ras transfectants but involves the development of cytogenetic changes. If such cytogenetic changes involve only gain in gene expression, then all hybrids formed by fusing highly metastatic v-Ha-ras RMC1 transfectants with the parental nonmetastatic RMC1 should be highly metastatic. If loss of a metastatic suppressor gene(s) is also involved, then such hybrids should be nonmetastatic since chromosomes from the nonmetastatic parental cells should supply the suppressor function. To test this possibility, a highly metastatic cloned v-Ha-ras transfectant was fused with the nonmetastatic parental RMC1 cells. Five hybrid clones were isolated that conserved the chromosomes from their parental cells. When these hybrid clones were injected into animals, primary tumors developed with the same tumor-doubling time as that of the highly metastatic parental v-Ha-ras transfectant (i.e., approximately 2 days). High metastatic ability was, however, suppressed in these hybrid clones. All hybrid clones continued to express v-Ha-ras p21. Thus, suppression of metastatic ability in the hybrids can occur even in the presence of an elevated v-Ha-ras p21 level. This suggests that the acquisition of metastatic ability following v-Ha-ras transfection involves loss of metastasis suppressor gene function in rat mammary cancer cells.

Quantitative variations in gene expression: possible role in cellular diversification and tumor progression

Changes in the quantitative expression of certain genes or in the amounts of their products can quickly stimulate progression to the metastatic phenotype. This has been done experimentally by transferring dominantly acting oncogenes such as c-H-rasEJ into susceptible cells or more recently by interfering with metastasis suppressor genes. In vivo such rapid qualitative changes in dominantly acting oncogenes or suppressor genes occur only rarely, and progression to highly metastatic phenotypes is thought to occur through a process involving the slow stepwise progression of a subpopulation of neoplastic cells to more malignant states. Such slow changes can be reversible and need not involve known dominantly acting oncogenes or metastatic suppressor genes, consistent with clinical and experimental observations on naturally occurring, highly advanced metastatic tumors. An important element in the natural progression of tumors to more malignant states may be their ability to circumvent host environmental controls that regulate growth and cellular diversity. They also evolve into heterogeneous cellular phenotypes, a process that appears to mainly involve quantitative changes in gene expression but can be rapidly stimulated in cell culture by the introduction of a dominantly acting oncogene or inhibited by the introduction of a suppressor gene. The oncogenes and suppressor genes that affect malignancy may control important steps in the quantitative regulation of sets of genes that are ultimately responsible for the cellular alterations seen in adhesion receptors, cell motility responses, cell-cell communication components, degradative enzymes and their inhibitors, growth factor receptors, components that aid in escape from host surveillance mechanisms and others that are important in malignancy.(ABSTRACT TRUNCATED AT 250 WORDS)

Gene expression and metastasis of somatic cell hybrids between murine fibroblast cell lines of different malignant potential

We have used somatic cell hybrids to study the relationship between ras sensitivity, metastasis, and the expression of ras-responsive or "metastasis-associated" genes. We have previously shown that NIH 3T3 cells are nontumorigenic, but are made metastatic by transfection and expression of activated ras (i.e., they are ras-sensitive). LTA cells, however, are initially tumorigenic, but nonmetastatic, and are not altered in malignancy by ras (i.e., they are ras-resistant). We also have shown that patterns of expression of ras-responsive and "metastasis-associated" genes differ markedly between these two cell types. In the present work, we have constructed three sets of somatic cell hybrids: NIH 3T3 X LTA cells (designated NL), NIH 3T3 X ras-transfected LTA cells (designated NLR), and LTA X ras-transfected NIH 3T3 cells (designated LNR). In all three sets of cell hybrids, pooled clones were found to be highly metastatic in the chick embryo assay, suggesting complementation had occurred. Those cell hybrids that contained ras (NLR and LNR hybrids) were significantly more metastatic than those that did not (NL hybrids). Selected clones of low and high metastatic ability from both NL and LNR fusions were examined for tumorigenicity and "experimental" metastatic ability in nude mice, as well as for expression of several genes thought to be involved in ras-induced progression and malignancy. Patterns of expression of these genes showed a relationship to level of malignancy of the hybrids and demonstrated a responsiveness to the expression of activated ras. These results suggest that the complementation of phenotype observed in the hybrids may arise through a gene regulatory factor(s) supplied by the NIH 3T3- to the LTA-derived parent.

Gene expression, cellular diversification and tumor progression to the metastatic phenotype

Alterations in the expression of certain genes or in their products can render benign tumor cells metastatic. Experimentally this has been quickly performed by transferring dominantly acting oncogenes such as c-H-rasEJ into susceptible cells, but in vivo such a rapid qualitative change in a dominantly acting oncogene occurs only rarely, and progression to highly metastatic phenotypes is thought to occur through a slow stepwise process. Such slow changes can be reversible and need not involve known dominantly acting oncogenes, consistent with clinical observations. An important element of the natural progression of tumors to malignancy may be their ability to circumvent microenvironmental controls that regulate growth and cellular diversity and to evolve into heterogeneous phenotypes, a process that appears to involve mainly quantitative changes in gene expression but which can be rapidly stimulated in cell culture by the introduction of a dominantly acting oncogene. It is proposed that the highly malignant cells that have slowly evolved in vivo with only a few qualitative gene changes have undergone extensive cycles of diversification and accumulation of quantitative changes in the expression of genes that encode products that are related to malignancy and metastasis. Thus, highly malignant cellular phenotypes can arise quickly through specific qualitative changes in critical controlling genes or more slowly by less critical qualitative genetic changes, coupled with cellular diversification and accumulation of quantitative changes in gene expression.

Adhesion molecules in lymphoma metastasis

Recently, many surface proteins of lymphoid cells that mediate adhesion to other cells and extracellular matrix have been identified. Several of these cellular adhesion molecules (CAM) are also expressed by metastatic lymphoma cells and may mediate adhesion to tissue components during the metastatic process. Correlations observed between expression of certain CAM, like MEL-14 and CD44, and particular patterns of spread, support this notion, but conclusive evidence is scarce. We have used T-cell hybridomas to study the mechanisms of wide-spread lymphoid metastasis. The results obtained with this model are reviewed here. The advantages are that a large number of genetically similar cell lines can be generated, which can be grouped in large panels of highly invasive and non-invasive cells. Invasiveness of these cells in hepatocyte and fibroblast monolayers correlates with experimental metastasis. Lymphoid CAM that are potentially involved in metastasis are reviewed. Several of these CAM are not, or not consistently, expressed by the invasive T-cell hybridomas, indicating that they are not indispensable. Notably, some of the CAM involved in the onset of an immune response or in migration into inflamed tissues, like ICAM-1 and VLA-4, and the 'homing receptors' MEL-14 and LPAM-1 do not seem to be involved. CAM that are consistently expressed by the T-cell hybrids include LFA-1, the beta-1 integrin subunit CD29, CD31 (PECAM-1) and CD44 ('Hermes homing receptor'). We have generated considerable evidence that LFA-1 is required for efficient metastasis of T-cell hybrids, based on the behavior of LFA-1-deficient mutants and revertants. High levels of LFA-1 are required. The relevant counterstructure is probably ICAM-2 rather than ICAM-1. Preliminary results suggest that also a beta-1 integrin, possibly VLA-5, plays a role. Finally, we summarize evidence indicating that CD31 and CD44 are primary candidates for involvement in metastatic spread of T-cell hybridomas.

Molecular oncogenetics of metastasis

Metastasis is a complex non-stochastic process that is most likely the result of genetic and epigenetic interactions of a wide variety of genes. The search for a single gene which can encompass such a pleiotropic response as to account for the observed phenotypic characteristics of metastatic tumour populations has been unsuccessful. Particular studies involving gene transfection, subtractive hybridisation and cell fusion are beginning to identify specific genes which contribute to metastasis in some cell types. However, such analyses are complicated by the inherent genetic instability and phenotypic heterogeneity present in tumour populations. A more detailed understanding of the metastatic process may require an abandoning of current generalised approaches to metastasis in favour of concentrating on key components of the metastatic cascade such as adhesion and invasion.

Analysis of the inhibition of tumour metastasis by sulphated polysaccharides

Lung metastases resulting from the intravenous (i.v.) injection of cells from the rat mammary adenocarcinoma 13762 MAT were significantly reduced by a variety of sulphated polysaccharides, the most effective being heparin, fucoidan and Carrageenan lambda. Although all the inhibitory polysaccharides were anticoagulants, it is unlikely that anticoagulation is the total explanation of their antimetastatic effect because: (i) heparin preparations from 2 different suppliers, although exhibiting comparable anticoagulant activities, differed 10-fold in their antimetastatic capability; (ii) certain sulphated polysaccharides consistently gave a 30% difference in the number of metastatic lesions, yet exhibited identical anticoagulant activity; and (iii) the entrapment of 13762 MAT cells in the lung was not impaired by heparin or fucoidan. It was more probable that the sulphated polysaccharides were interfering with the passaging of tumour cells across the capillary wall as heparin significantly inhibited metastasis when injected up to 3 hr after lodgement, and heparin and fucoidan caused a gradual loss of tumour cells from the lung which only became apparent greater than 1 hr following cell lodgement. The data did not eliminate the possibility that tumour cell adhesion to the endothelium occurred via sulphated polysaccharide recognition. A negative correlation existed between the sulphated polysaccharides that bound to the surface of the tumour cells and those that inhibited metastasis.

Sporadic somatic fusion between MDAY-D2 murine tumor cells and DBA/2 host cells: role in metastasis

An ouabain- and 6-thioguanine-resistant mutant (K3T103) of the metastatic MDAY-D2 murine tumor cell line was transplanted s.c. into syngeneic DBA/2 mice in order to isolate K3T103 X host cell hybrids emerging in vivo, and examine their metastatic potential. Of 10 tumor-bearing animals that were analyzed at various time intervals, only 3 developed fusion products in the primary site, present at a frequency ranging between 10(-5) and 10(-4). These hybrids survived in HAT + ouabain medium, had a lymphoblastoid morphology and a hypo-tetraploid karyotype, were non-adherent, and were as rapidly metastatic as K3T103 cells when transplanted s.c. or i.v. into DBA/2 mice. All these characteristics were shared by cloned hybrids generated in vitro following polyethylene glycol (PEG)-mediated fusion of K3T103 cells with normal DBA/2 normal splenocytes. In marked contrast, the products of K3T103 X DBA/2 normal lung fibroblast fusion displayed a fibroblastic appearance, were adherent, progressively ceased to divide and remained dormant for several weeks in culture. These results indicate: that spontaneous fusion of K3T103 cells with host cells in the course of their expansion and subsequent dissemination is a stochastic and rare event, and that since the expression of their tumorigenic and metastatic potential is retained after fusion with splenocytes, host cells of lymphoreticular origin are most likely involved in that process.

Selective suppression of metastasis but not tumorigenicity of a mouse lung carcinoma by cell hybridization

Somatic cell hybrids were produced by polyethylene glycol-induced cell fusion between metastatic CMT 167 (HGPRT-/OUAR) C57BL/Icrfat mouse lung carcinoma cells and 2 non-metastatic cell lines: C3H/He mouse L-M(TK-) cells of mesenchymal origin and EJ (OUAS) human bladder carcinoma cells. Fusion of 2 different CMT167 (HGPRT-) clones with L-M(TK-) cells followed by selection in HMT medium gave rise to 14 intraspecific hybrids, which were shown to express H-2 antigens specific for both the C57 and C3H mouse strains. Three interspecific hybrids arising from fusion of EJ(OUAS) and CMT167(HGPRT-/OUAR) cells were selected in HMT/ouabain medium and characterized by human isozyme analysis. All the hybrids produced large tumours after subcutaneous inoculation of 5 X 10(5) cells into adult athymic nu/nu mice. The intraspecific hybrid tumours were predominantly sarcomatous (mesenchymal) in structure but a few contained epithelial acini. Metastatic ability (as assessed by production of lung metastases) was completely suppressed in 13 of the 14 mouse/mouse hybrid cell clones. These results suggest that tumorigenicity, tumour structure and the ability to metastasize are expressed independently. The interspecific hybrids, which had not retained a full human chromosome complement, produced metastatic tumours that remained epithelial in structure.

Purification and characterization of an inhibitor of plasminogen activator released by rat mammary adenocarcinoma cells

An inhibitor of plasminogen activator (PA) secreted by a tumorigenic, but non-metastatic, rat mammary adenocarcinoma cell line has been purified to apparent homogeneity and characterized. It strongly inhibited human urokinase, but was 100 times less potent in inhibiting bovine trypsin and had no effect on plasmin or thrombin. A secreted, urokinase-type PA (Mr 48 000) and a cell-associated PA from a metastatic rat adenocarcinoma cell line were also strongly inhibited. In contrast, a tissue-type PA (Mr 66 000), secreted by human melanoma cells, was only slightly inhibited. Purified inhibitor showed a band of Mr 66 000 in sodium dodecyl sulphate/polyacrylamide gel electrophoresis and an isoelectric point of 4.5 after chromatofocusing. The inhibition of human urokinase was non-competitive.

Dormant tumor cells in liver and brain. An autopsy study on metastasizing tumors

In 96 of 100 autopsy cases with metastasizing tumor diseases single disseminated tumor cells were detected in liver or brain tissues. These tumor cells appear to be in a resting state. Most of the resting or dormant tumor cells (dtc) are distinguishable from tumor cells within metastasis (mtc) by morphological criteria such as cell size or density, shape and size of the nucleus. These observations suggest that disseminated cells from many primary tumors might pass through a resting (Go) state before they eventually initiate malignant cell growth and give rise to metastatic colonies. Most likely the dtc are not affected by systemic chemotherapy.

Quantitative genetic analysis of tumor progression

Metastasis and resistance to chemotherapy are common features of progressed cancers. With respect to the latter phenotype, it is thought that during tumor growth drug-resistant cells arise spontaneously at rates characteristic of the genetic alterations involved. On application of chemotherapy, such variant tumor cells are more likely to survive, and they may eventually dominate, resulting in a non-responsive malignancy. Aspects of this model have been confirmed in a number of experimental systems and in patients. In contrast to our understanding of drug resistance, steps involved in the progression to metastatic spread of tumor cells are much less well-understood. In this review we describe methodologies of quantitative genetic analysis with reference to development of drug resistance. We then describe attempts by ourselves and others to use a similar approach to investigate metastatic properties. Based on these studies, we have proposed the quantitative 'dynamic heterogeneity' model of tumor metastasis, which is presented here. Using an 'experimental' metastasis assay and Luria-Delbruck fluctuation analysis, we determined that in murine KHT fibrosarcoma and B16 melanoma lines, 'metastatic' variants with a distinct phenotype are generated at high rates. These variants are relatively unstable resulting in a dynamic equilibrium between generation and loss of metastatic variants. The metastatic ability of such a tumor population is thus dependent on the frequency of a subpopulation of metastatic variants which are turning over rapidly. This dynamic heterogeneity model is able to quantitatively provide a unifying explanation for a wide range of observations concerning tumor heterogeneity and clonal instability. Genetic mechanisms involving rapid rates have been characterized in drug-resistant variants. We speculate that similar processes may be involved in different aspects of tumor progression such as those resulting in metastasis.

Studies on rat mammary adenocarcinomas: a model for metastasis

Studies carried out by the authors on the rat mammary adenocarcinoma cell lines MAT 13762 and DMBA-8 are summarized. A series of variants and somatic cell hybrids have been prepared and partially characterized in terms of phenotypic properties which may correlate with metastatic potential. These include measurement of in vitro migration, lectin binding properties, expression of procoagulant activity and shedding of cell surface components. Particular emphasis has been placed on the production of enzymically-active plasminogen activator, as this seems to correlate with the ability of cells to metastasize. The finding has also been made that several of the cell types studied produce, in vitro, an inhibitor of plasminogen activator which may influence the metastatic behaviour of tumor cells. Results obtained are discussed in the context of the usefulness of these tumor systems for the study of spontaneous and experimental metastasis and the factors involved in these processes. Preliminary results of cloning and fluctuation analysis of metastatic potential together with discussion of the role of the metastatic heterogeneity and the formation of metastatic variants by mutation events are included.

Somatic cell fusion as a source of genetic rearrangement leading to metastatic variants

Tumor cell populations displaying metastatic properties often have higher gene dosage than their less malignant progenitor tumors, as shown by increased ploidy levels, chromosome duplication and gene amplification. The acquisition by tumor cells of high chromosome numbers may be due to endoreduplication or somatic hybridization either between tumor cells or between tumor and host cells. All such mechanisms increase genetic variability and instability in tumor cells since they trigger a polyploidization-segregation cycle. Among the wide variety of segregants which may emerge from high-ploidy cells, variants with increased malignancy can be positively selected in vivo. Evidence for in vivo fusion of tumor and normal host cells has been reported in different tumor systems. However the attainment by tumor-host hybrids of a higher degree of malignancy has only been observed following substantial chromosome segregation. The involvement of a cell of bone marrow origin as preferential host partner in the fusion process has been proved both by studies on tumor-host hybrids in bone marrow radiation chimeras and in vitro hybridization experiments between non-metastatic tumors and normal lymphoreticular cells which have led to the establishment of metastatic variants. Several different segregational mechanisms may bring about homozygosity or hemizygosity of recessive alleles in tumor-host hybrids, leading to their expression. The marked chromosome dynamics of tumor-host hybrids are also responsible for extensive chromosome rearrangements. At the molecular level these may represent mechanisms causing altered oncogene activity. The activation of new oncogenes by transposition or amplification as well as the amplification of previously activated oncogenes are the mechanisms most likely to be responsible for transition from low to high malignancy, occurring through ploidy changes, such as those produced by somatic mating.